In recent years, many different techniques for the fast production of three-dimensional models have been developed for industrial use. These solid imaging techniques are sometimes referred to as rapid prototyping and manufacturing (“RP&M”) techniques. In general, rapid prototyping and manufacturing techniques build three-dimensional objects layer-by-layer from a working medium utilizing a sliced data set representing cross-sections of the object to be formed. Typically, an object representation is initially provided by a Computer Aided Design (CAD) system.
Stereolithography, presently the most common RP&M technique, was the first commercially successful solid imaging technique to create three-dimensional objects from CAD data. Stereolithography may be defined as a technique for the automated fabrication of three-dimensional objects from a fluid-like photopolymer build material utilizing selective exposure of layers of the material at a working surface to solidify and adhere successive layers of the object (i.e. laminae). In stereolithography, data representing the three-dimensional object is input as, or converted into, two-dimensional layer data representing cross-sections of the object. Layers of photopolymer build material are successively formed and selectively transformed or solidified (i.e. cured) using a computer controlled laser beam of ultraviolet (UV) radiation into successive laminae according to the two-dimensional layer data. During transformation, the successive laminae are bonded to previously formed laminae to allow integral formation of the three-dimensional object. This is an additive process. More recently, stereolithographic designs have employed digital light-processing technology wherein visible light initiates the polymerization reaction to cure the photopolymer build material (i.e. also referred to as resin).
Stereolithography represents an unprecedented way to quickly make complex or simple parts without tooling. Since this technology depends on using a computer to generate its cross-sectional patterns, there is a natural data link to CAD/CAM. Such systems have encountered and had to overcome difficulties relating to shrinkage, curl and other distortions, as well as resolution, accuracy, and difficulties in producing certain object shapes. While stereolithography has shown itself to be an effective technique for forming three-dimensional objects, other solid imaging technologies have been developed over time to address the difficulties inherent in stereolithography and to provide other RP&M advantages.
These alternate technologies, along with stereolithography, have collectively been referred to as solid freeform fabrication or solid imaging techniques. They include laminated object manufacturing (LOM), laser sintering, fused deposition modeling (FDM), and various ink jet based systems to deliver either a liquid binder to a powder material or a build material that solidifies by temperature change or photocuring. Each of these additive technologies have brought various improvements in one or more of accuracy, building speed, material properties, reduced cost, and appearance of the build object.
During the same time period that solid imaging or solid freeform fabrication has evolved, the two-dimensional imaging industry evolved ways to displace the projected image on a screen or, in the case of the printing industry, on a receiving substrate. These approaches addressed the basic problem that digital light projectors produce images with coarse resolution. Digital light projectors typically project only 100 pixels per inch for an image size of 10.24 inches by 7.68 inches, so their resolution is limited by the pixel sizes. The photographic printing industry especially has employed techniques to shift two-dimensional images to improve resolution by a variety of techniques, including moving the light source or light valve. Other approaches have included moving or shifting the photographic paper, using polarizing and double refracting plates, and, in the case of image projection systems, using multiple spatial light modulators. All of these systems have addressed the inherent limitation of image distortion when projecting resized digital images or the problem of light valve projectors, such as a liquid crystal display (LCD) or a digital micro-mirror device (DMD), having a fixed number of pixels. Attempting to utilize image displacement techniques with digital image projections in solid imaging applications presents unique problems because of the three-dimensional aspect of the object being created. The problems of two-dimensional digital image projection, when applied to three-dimensional solid imaging, cause inaccurate feature placement, potential loss of feature details, and smoothness of curves or edges on objects being built to be roughened or uneven and poorly defined. Most recently, techniques have been developed using pixel shifting to address this problem. However, those approaches suffer from the deficiencies of requiring multiple exposures of individual pixels, thereby inherently slowing the process, and requiring mechanical hardware to accomplish the pixel shifting. Additionally, when using multiple exposures with techniques such as pixel shifting, there are alignment issues that must be addressed to ensure the exposures are properly positioned to obtain maximum resolution and the desired edge smoothness.
Lastly, none of the prior solid freeform fabrication approaches, while making substantial improvements, achieve a truly low cost system that produces highly accurate and visually appealing three-dimensional objects in a short build time.
These problems are solved in the design of the present invention by combining a new solid imaging technique with the use of digital imaging projection in a manner that provides accurate object features while achieving high resolution and object wall smoothness in three-dimensional object fabrication while being relatively low cost and not requiring additional hardware.